1. Field of the Invention
The present invention relates to optical devices and, more specifically, to an optical interferometric sensor.
2. Description of the Prior Art
Sensors for detecting and measuring absolute or relative values of physical quantities such as chemical or biochemical concentration, magnetic or electric field strengths, pressure, strain, temperature, and pH, for example, in an environment to which the sensor is exposed, are well known in the art. Prior art sensors include optic sensors which provide measured values directly or by means of transducers. Interferometric type optic sensors are highly sensitive. Such sensors employ an interferometer to provide information about a condition sensed. An interferometer is an instrument that splits light from an input source into two light beams and, after the light beams are caused to travel through different paths, recombines the two beams resulting in interference and an interference pattern. An analysis of the interference pattern provides a sensitive measure of the difference in effective path length of the two optical paths.
Integrated optic sensors are monolithic structures characterized by the integration of various optical components into a single optic waveguide construction. An integrated optic sensor is typically a thin-film device comprising a waveguide constructed on a single substrate, which generally provides other optical elements or components to diffract, refract or reflect different beam portions propagating in the waveguide for purposes of separating or combining them. Integrated optic technology is particularly useful in providing the optical elements heretofore associated with interferometric sensors employing separate and discrete optical components. The prior art now includes integrated optic sensors that incorporate a variety of components including lenses, sensing fields, and filters on a single substrate.
A typical integrated optic sensor comprises one or more channel waveguides fabricated as a planar construct on a substrate. A channel waveguide is a linear structure of typically small cross-section, on the order of several micrometers wide by several micrometers high, providing an optical path for a propagating light beam. The index of refraction of the channel waveguide is higher than the index of refraction of the surrounding or supporting substrate. A light source and possibly a coupling mechanism are provided to cause a light beam to propagate within the channel waveguide. The light source can be a laser, a light emitting diode (LED), or an incandescent light source. The propagating light beam passes through a sensing region of the channel waveguide which is reactive to particular conditions of the environment. The environment may cause changes in the propagation characteristics of the channel waveguide, such as a change in the refractive index. The change in the refractive index changes the effective optical path length through the channel region, thereby changing the phase of the light beam as it emerges from the channel waveguide. Alternatively, if the channel waveguide is not directly sensitive to a particular environment, it may be coated with a material that is reactive to the environment, or to a component thereof, causing a change in the refractive index the light beam encounters. An optical output beam from the sensor can therefore be used for measuring the relative or absolute value of the condition of the environment.
The optical input beam propagates through the waveguide in modes which satisfy Maxwell""s equations. Maxwell""s equations govern the electric and magnetic fields of an electromagnetic wave propagating through a medium. The modes may be characterized by the frequency, polarization, transverse field distribution and phase velocity of the constituent waves. In rectangular channel waveguides the modes are designated as TEm,n and TMm,n which are orthogonally polarized components of the light beam, transverse electric and transverse magnetic, respectively, with mode number indices m and n taking non-negative integer values. Each mode represents a different field distribution corresponding to the number of wave nodes across the waveguide in each direction. The allowed modes are determined in part by the configuration of the boundaries of the waveguide, which for integrated optic sensors are the interfaces between the substrate and waveguide, the environment and the waveguide, and the coating and the waveguide. Depending on the boundaries, the waveguide materials, the waveguide dimensions and the wavelength of the input light source, no modes, one mode, or more than one mode may be allowed to propagate through the waveguide.
Commercially available integrated optic interferometers include those utilizing a Mach-Zehnder interferometric technique. This technique is characterized by single mode propagation of two light beams through two light paths, then combining the two beams to produce an optical interference pattern. Generally, a Mach-Zehnder device receives a single input light beam which is then split by a beam splitter into two beams that are directed through two different channel waveguides. Changes in the optical path length of one of the waveguides are effected when the environment causes a change in its refractive index. The beams emerging from the channel waveguides are recombined to produce a single interfering beam which is indicative of the relative or absolute change caused by exposing the device to the environment.
Integrated optic interferometers employing the Mach-Zehnder configuration provide outstanding sensitivity and can be made in small sizes. These sensors, however, suffer in that they rely on two or more single-mode channel waveguides with typical cross-sectional dimensions of 0.1 xcexcm by 3 xcexcm each, making fabrication difficult and costly. Most importantly, the small size of the channels make efficient light coupling difficult to achieve with Mach-Zehnder interferometers. The light coupling difficulty makes this type of interferometer all but useless for many applications.
A second type of integrated optic interferometric sensor uses a planar waveguide as the planar construct. A planar waveguide is defined by only two (parallel) boundaries, rather than the four rectangular boundaries typical of a channel waveguide. In a planar waveguide, the propagating modes are designated as TEm and TMm (transverse electric and transverse magnetic, respectively), with the mode number index m taking non-negative integer values. As in the channel waveguide, the boundaries, the waveguide materials, the waveguide dimensions and the wavelength of the input light source determine whether no modes, one mode, or more than one mode may be allowed to propagate through the waveguide.
The descriptions herein of the prior art and of the invention use the term xe2x80x9copticxe2x80x9d and xe2x80x9clight,xe2x80x9d but it must be recognized that the techniques described are phenomena of electromagnetic radiation in general. Thus, the term xe2x80x9copticxe2x80x9d and xe2x80x9clightxe2x80x9d herein should be read as referring to any electromagnetic radiation that meets whatever constraints are imposed by the characteristics of the various components of the sensor (such as the dimensions of the optical path) and the nature of the interaction between the sensor and properties of the environment to be sensed (such as the sensitivity of the sensor as a function of wavelength). Typically, the light will be in the visible or near-visible wavelength range.
Because prior art systems usually expose the active components of the sensor to different parts of the environment, in homogeneities in the environment can give rise to inaccurate results. For example, in a solution of a substance dissolved in a liquid one region may have a first concentration whereas an adjacent region may have a different concentration. Inaccuracies in the results of the sensor""s analysis can result from the two arms of the interferometer probing regions with different concentrations. Furthermore, prior art systems provide no way to ensure that a consistent sample volume is being interrogated, which can give rise to inconsistent results. Thus, the prior art has the additional disadvantage of not constricting the portion of the environment being analyzed to a definite volume.
The disadvantages of the prior art are overcome by the present invention which, in one aspect, is an interferometer for detecting a property of an environment. The interferometer includes a source of a beam of light and a first planar waveguide. The first planar waveguide has a first end, an opposite second end and a first interior surface. A first coupler is disposed adjacent the first end so as to be capable of coupling a first portion of the beam into the first planar waveguide. A second coupler is disposed adjacent the second end so as to be capable of de-coupling a second portion of the beam from the first portion of the beam and onto a first predetermined exit path. A first region is disposed along the first interior surface between the first coupler and the second coupler. The first region allows light to propagate therethrough as a first function of exposure to an environment disposed adjacent thereto. The interferometer also includes a second planar waveguide having a third end, an opposite fourth end and a second interior surface. The second planar waveguide is disposed substantially parallel to the first planar waveguide so that a first portion of the first interior surface and a second portion of the second interior surface define a cavity therebetween. A second region is disposed along the second interior surface, between a third coupler and a fourth coupler, that allows light to propagate therethrough as a second function, different from the first function. The third coupler is disposed adjacent the third end so as to be capable of coupling a third portion of the beam into the second planar waveguide. The fourth coupler is disposed adjacent the fourth end so as to be capable of de-coupling a fourth portion of the beam from the third portion of the beam and onto a second predetermined exit path. At least a portion of the second predetermined path is co-incident with at least a portion of the first predetermined path so as to form a combined beam. A phase difference detector that is responsive to the combined beam indicates a phase difference between the second portion of the beam and the fourth portion of the beam, so as to indicate a property of the environment.
Another aspect of the invention is a method of manufacturing an interferometric sensor. A first waveguide is applied onto a first surface of a first substrate so that the first waveguide has a first end and an opposite second end. A first coupler is disposed adjacent the first end and a second coupler adjacent the second end. A first treatment is applied to a portion of the first waveguide. The first treatment allows. light to propagate through a portion of the first waveguide as a first function of exposure to a property of an environment adjacent the portion of the first waveguide. A second waveguide is applied onto a second surface of a second substrate so that the second waveguide has a third end and an opposite fourth end. A third coupler is disposed adjacent the third end and a fourth coupler adjacent the fourth end. The first substrate is disposed substantially parallel to the second substrate so that the first coupler and the third coupler both lie on a first common optical path and so that the second coupler and the fourth coupler both lie on a second common optical path. The first common optical path and the second common optical path are at an angle to the direction of the first waveguide and the second waveguide. The first surface and the second surface face each other and define a cavity, having a width, therebetween.
Other aspects of the invention are methods of supporting analysis of a portion of an environment in an interferometric sensor in which the portion of the environment is subjected to an oscillating field that influences one or more components of the environment during the analysis.
These and other aspects of the invention will become apparent from the following description of the preferred embodiments taken in conjunction with the following drawings. As would be obvious to one skilled in the art, many variations and modifications of the invention may be effected without departing from the spirit and scope of the novel concepts of the disclosure.